Generally, a metal can type secondary battery is the most dominant type of secondary battery, and in many cases, this battery is produced in such a manner that, after inclusion of a power generating element and an electrolyte in a metal can, a step of extracting electrodes is performed and lastly, a metal cover is laser welded. Thus, the can type secondary battery has a high sealing property, being advantageous in having a very few problems in terms of leakage of an electrolyte from a metal can to outside, even in a long-term use. Meanwhile, there has been a strongly demand for an improved weight density as well as shape flexibility, being attributed to the recent appearance of an increasing number of secondary batteries having a laminate film as an armored body which are lighter than the metal can type secondary battery and have shape flexibility.
A laminate film used a laminate film secondary battery is obtained by bonding a nylon sheet to the front surface of a thin aluminum sheet, and adhering a polyethylene sheet or polypropylene sheet, in general, having a fusion bonding property, to the rear surface of the thin aluminum sheet. After wrapping a power generating element with this aluminum laminate film and extracting electrodes, three or four sides of the laminate film are fusion-bonded to produce a secondary battery, which is often called a laminate film secondary battery. Because the laminate film type secondary battery structurally has fusion-bonded sides, the sealing property of the fusion-bonded sides can be said to be related to the electrolyte leakage problem. Under present circumstances, however, it has been difficult that the sealing property obtained by fusion bonding secures longer-term reliability than the sealing property obtained by laser welding in the can type battery, and it has therefore been a large challenge for the laminate film type secondary battery to extend the life of the sealing property.